The invention relates to an electrically conductive elastomer mixture comprising a thermoplastic styrene elastomer and, as an electrically conductive filler, metal-containing particles.
The invention further relates to a method of manufacturing an electrically conductive elastomer mixture, the elastomer mixture comprising a thermoplastic styrene elastomer and, as an electrically conductive filler, metal-containing particles.
The invention still further relates to a use of an elastomer mixture according to any one of claims 1 to 8 in an electrically conductive seal.
As to their electric properties, polymers and polymer mixtures are usually dielectrics. In some applications, however, it is advantageous for the polymer material to be to some extent electrically conductive. Such applications include e.g. antistatic packages, containers and pipework for inflammable substances, electrostatically paintable formed pieces, and many other applications known per se. Furthermore, the constantly increasing number of electronic devices has resulted in the fact that, on one hand, their electronics have to be protected from electromagnetic interference caused by other electronic devices and, on the other hand, electronic interference caused by such devices on the outside has to be reduced. In other words, the devices have to be EMI-shielded. Electric conductivity is often achieved by compounding a substantially electrically non-conductive polymer operating as a matrix material with metal or metal-covered particles, carbon or graphite or combinations thereof operating as a conductive filler.
The term elastomer refers to a material consisting of macro-molecules and characterized by extensibility and rapid recovery to the original shape after release of tension. Electrically conductive elastomer mixtures are known wherein the matrix material is a thermosetting material, e.g. a silicon polymer. Such a matrix material has to be cross-linked in order to achieve elastic properties and, in general, to make a product workable. Cross-linking is very energy- and time-consuming, and it also requires special cross-linking equipment, making products slow and expensive to manufacture. Prior art also includes electrically conductive thermoplastic elastomer mixtures. Compared to the thermosetting materials, these materials are quick and inexpensive to process, but their specific resistance values are often higher than those of the thermosetting materials.
In order for an elastomer mixture to be electrically conductive, the electrically conductive particles compounded therewith have to be in contact with one another, or a distance between the particles has to be small enough to enable an effective tunneling current to flow therebetween. Furthermore, the particles should form chains leading through the matrix material (G. R. Ruschau et al., J. Appl. Phys. 72, 1992, pp. 953 to 959). The volume fraction of the electrically conductive particles has to be large enough for the aforesaid condition to be met. However, increasing the volume fraction of the filler impairs (inter alia) the mechanical properties, processability or surface quality of the elastomer mixture. In addition, the material often becomes much more expensive. Therefore, the volume fraction cannot be increased infinitely in order to improve electric conductivity.
It is known to improve the electric conductivity of electrically conductive fillers by treating the surface of particles in different manners. An alternative is to employ direct coating of the particles with an electrically conductive polymer. Nickel particles, for example, have been coated with polypyrrole by using a method wherein sodium dodecyl sulphate (SDS) was first applied onto the surface of the nickel particles (Genetti W. B. et al., J. Mater. Sci. 33, 1998, pp. 3085 to 3093). SDS is a surface active agent which forms a double layer around the nickel particles. Next, the pyrrol was subjected to polymerization inside the double layer. This considerably improved the specific conductivity of a particle-filled polyethylene.
U.S. Pat. No. 6,875,375 discloses an electrically conductive thermoplastic elastomer mixture comprising an elastomer matrix and, as an electrically conductive filler, metal-coated particles. The electrically conductive particles are at least partly coated with a self-assembled molecule layer. Optionally, coating may contain molecule conductors which settle between the self-assembled molecules. The specific resistance of the elastomer mixture is low, and does not substantially increase due to the influence of compression.
Addition of stearic acid has also been found to have effects similar to those of the coating materials disclosed in U.S. Pat. No. 6,875,375.
However, the known electrically conductive thermoplastic elastomer mixtures involve a significant problem. Namely, such mixtures are often used for the manufacture of seals. Typically, seals are elongated objects which often form at least one closed loop such that their cross-section is small compared to their length. When such a product is manufactured by injection molding, the molten elastomer mixture is forced to flow a long distance in a narrow channel which constitutes a mold cavity. In other words, inside a mold, the flow distance between a gate, through which the elastomer mixture is fed into the mold cavity, and a weld line, i.e. the point in the mold cavity at which the material flows that fill up the mold cavity meet one another, is long. Consequently, the elastomer mixture is subjected to large shear forces during the entire process of the mold being filled up. As is widely known, an elastomer mixture flows in a channel such that the flow rate of the flowing elastomer mixture is at its lowest in the vicinity of the surface of the channel, increasing therefrom upon approaching the centre line of the cross-section of the channel. In such a flow situation, an elastomer mixture containing a solid filler often becomes subjected to non-homogenization such that some particles that actually belong to the part of the elastomer mixture which flows in the vicinity of the surface of the channel or which has already attached to the surface become separated from this part and propagate together with the mixture flowing in the inner parts of the channel. Due to this phenomenon, the consistency of the electrically conductive filler of the elastomer mixture in the vicinity of a gate may be substantially lower than that of the electrically conductive filler of the elastomer mixture fed into the mold. This substantially increases the value of electric resistance in the vicinity of the gate, which is why the seal fails to fulfill the requirements set for its electric conductivity.